1. Field of the Invention
The invention generally relates to a Moiré deflectometer comprising at least two non-mechanical, transparent, spatial light modulators for demonstrating two sets of microscopic parallel lines on two parallel planes on two of the modulators thereby creating a Moiré fringe pattern and a method for using the same. More particularly, each of the spatial light modulators may be a liquid crystal display, an electrochromic device, a micromirror array, a microlouvre array, an electro-optic device, or a holographic device.
2. Description of Related Arts
The use of the basic ruled pattern to aid in testing of optical components, such as mirrors, was conceived by the Italian Vasco Ronchi in 1923. He used a ruled grating pattern of fine parallel lines to test the deviation of a mirror from its correct figure. In his experiments, the ruled grating pattern was put in front of the eye after removing the eyepiece from a telescope to visually assess the mirror quality. This was done by pointing the telescope at a star and viewing the deviations of the ruling grids from a straight line.
Moiré deflectometry involves the use of multiple mechanically ruled, fixed-period Ronchi gratings composed of a reflective or absorptive material deposited on a transparent substrate in straight lines that are spaced at uniform distances from one another. Two of these rulings are placed in the optical path of a surface under test. The shadow of the first Ronchi ruling is superimposed on the shadow of the second Ronchi ruling to produce Moiré fringes, which contain information about the mechanical and optical figure of the surface under test. From the Moiré fringes, it is possible to calculate the slope at every point by computing the tangent of the deflection angle. The deviations along each fringe make it possible to obtain a map of the slopes over the entire reflective surface. Using this technique, the mapping of the slopes is one-dimensional. In order to determine the slopes in the other dimension, the rulings must be rotated 90° from their original position.
FIG. 1 and FIG. 2 illustrate the classical approach of a Moiré deflectometer design, as described in U.S. Pat. Nos. 4,459,027 and 4,810,895, which requires different arrangements for testing phase objects and reflective objects. A phase object is typically characterized as a transparent object that changes the phase of light as it passes through the object. Examples of phase objects are lenses, variation in the density of liquids, and thermal variations in the atmosphere. In the basic arrangement for testing phase objects as illustrated in FIG. 1, collimated beam passes through the phase object to be examined and then traverses the set of gratings G1 and G2 of identical pitch, p, separated by a distance Δ. Each grating is a piece in which numerous microscopic parallel lines are scribed. A Moiré pattern is formed by overlapping the shadow of the first grating with the shadow of the second grating on a mat screen attached to the grating G2. To analyze a reflective surface, such as a mirror, the arrangement is modified as illustrated in FIG. 2 in which the collimated beam is first projected onto a specular surface at an angle θ, and the reflected beam passes through the gratings G1, G2. The equations of the Ronchi rulings for G1 and G2 can be written as: y+f1 (x, y)=n p and y+f2(x, y)=m p where p is the pitch of the lines and n=1, 2, 3 . . . and m=1, 2, 3 . . . . When the difference between the two distorted gratings are superimposed to form a Moiré pattern and the substitution l=m−n is made, the result is: f1 (x, y)−f2(x, y)=l p. The arrangement suffers from an inherent distortion l×cosθ in one axis. Other interfering effects, such as shadowing, might occur at higher angles.
The arrangement was further developed in U.S. Pat. No. 4,810,895, as shown on FIG. 3 and FIG. 4. The basic arrangement for measuring a phase object is shown in FIG. 3 and comprises a point source light, which produces a diverging beam of light. The diverging beam of light passes through a beam splitter and is directed to an optical system, which includes an objective lens L1, the phase object and a mirror. The light from the diverging beam is collimated by lens L1, passes through the phase object and reflects off of the mirror and back through the phase object. The light returns to the beamsplitter and is directed to a second objective lens L2, where it is collimated and traverses the set of gratings G1 and G2. A Moiré pattern is formed by the overlap of the shadows of the first grating with the second grating and is viewed on a mat screen. By analogy with the discussion regarding U.S. Pat. No. 4,459,027, the arrangement shown in FIG. 3 can be modified as shown in FIG. 4 to measure specular objects, such as curved mirrors by replacing the flat mirror and objective lens L1 with a test mirror. In FIG. 4, a point source of light, such as a laser, produces a diverging beam of light, which, after passing through a beam splitter is directed to the optical system as the one in FIG. 3 that retraces the light in the form of a converging beam from the examined object back towards the point source. The major difference is that FIG. 4 does not contain the large objective lens L1 shown in FIG. 3. In FIG. 4, the curved specular surface provides the same function of the objective lens L1 in FIG. 3.
An article published in 1979 by O. Kafri, “Tunable moiré grating for optical mapping,” Opt. Lett. 4, 314–316, provides methods to change the instrument resolution by changing the spacing of the rulings and their pitch. The article suggests that mechanical linear or rotary motions of the Ronchi rulings or replacement with rulings of a different pitch can change the resolution of the Moiré deflectometer. However, either of these methods can compromise the critical optical alignment of the Ronchi rulings with other instrument optics and adversely impact the accuracy of the measurements.
There is a need for eliminating the limitations with Moiré deflectometry caused by the fixed Ronchi gratings thereby providing Moiré deflectometry surface measurements over a large dynamic range from one meter to ten micrometer spacial period bandwidth without image re-registration, mechanical adjustment, or software alignment.
Rather than any mechanically ruled gratings, U.S. Pat. No. 6,392,754 uses a light grid of parallel lines produced by a light source and a physical grid by a matrix composed of a large number of LEDs. The light grid is projected on a curved surface, such as an auto windshield, and then captured in a camera to be compared with a stored grid so as to determine the surface conditions. However, the projected grid does not contain information that can be related to a quantitative description of the surface contour. Further, the comparison requires a careful alignment between the captured grid and the stored grid.
The article by Sansoni et al, “A Novel Adaptive System for 3D Optical Profilometry Using a Liquid Crystal Light Projector,” (IEEE Transactions on Instrumentation and Measurement, VOL. 43, No. 4, August 1994) provides a 3-D optical whole-field profilometer based on adaptive projection of one Ronchi grid by means of a Liquid Crystal Display (LCD) unit for industrial dimensional analysis, such as 3-D contouring and gauging of large-surface car parts, or fast dimensional analysis of objects in relation to recognition of targets by robots. The profilometer requires a well-defined geometric pattern between the entrance and exit pupils of the projection and imaging optics, and the pupil profile of the object is evaluated with respect to a well-defined reference surface. Demodulation is applied to the image of the grating deformed by the reference plane to obtain the reference phase map. Such a reference phase map must be acquired every time the profilometer is set up or calibrated. In addition, to change the profilometer sensitivity, the pitch of the LCD Ronchi ruling must be changed. The LCD projector can generate a coarse grid and a fine grid to increase resolution. The grating is varied in contrast and in period to adapt to the shape of the object under measurement.
There is a need for applying a light grid in Moiré deflectometry to define the surface contour of the object without a reference grid, a set of reference fringes, or a known geometric relationship between the instrument and the object. There is also a need to adjust the resolution of a Moiré deflectometer without mechanically changing the pitch of the lines of a ruling or moving the physical position of a ruling.
The article by Takacs et al, —“Surface Topography Measurements Over One Meter to TenMicrometer Spacial Period Bandwidth,” SPIE vol. 1164, describes a method for measuring the surface contour of an object in the direction along the surface, rather than normal to the surface.
The article by O. Kafri and A. Livnat, “Reflective surface analysis using Moiré deflectometry,” Appl. Opt 20, 3098–3100, 1981, describes using Moiré deflectometry to measure ray deflections from reflective surfaces. The article suggests that the sensitivity can be changed by varying the spacing between gratings but does not provide a method for doing so. Moreover, the article describes that the stability requirements are limited by the sensitivity of the measurement, which can be compromised by simply changing the location of the gratings through mechanical means.